Trunked communications systems are known in the art. Such systems typically include at least one central control unit that controls channel allocation as between various subscriber units (as used herein, "subscriber units" includes all remote transceiving devices, such as mobile units installed in vehicles, other control stations, portable devices, and RF linked telephones). To accommodate range requirements nd facilitate inter-unit communications, such systems also usually include two or more repeater stations that function to rebroadcast (or "repeat") incoming received messages on communications channels as assigned by the central control unit.
Once the central control unit has assigned a communications channel to a subscriber unit, normal voice communications can be carried out. To prevent the channel from being reassigned by the central control unit, the subscriber unit will typically transmit a sub-audible connect tone (A) in parallel with the voice transmission (B) as depicted in FIG. 1. The central control unit will sense the presence of the connect tone (A) and maintain the channel assignment.
To further aid in controlling the communications process, the central control unit will continuously transmit a low speed handshake signal (C) in parallel with voice transmissions (B) as depicted in FIG. 2. The subscriber units can receive and detect this low speed handshake signal (C) and operate as desired in a predetermined fashion (for example, this signal can be used to unmute the audio processing circuitry of the subscriber units). Also, when communications are concluded, the transmitting subscriber unit transmits a disconnect signal on the communications channel to the central control unit. Upon concluding a hang-time period, the central control unit transmits a system disconnect signal to all relevant subscriber units to terminate the channel assignment.
Secure communications systems are also known in the art. Such systems typically render a voice message unintelligible to prevent unauthorized reception. To accomplish this, the voice message can be digitized and processed through an encryption device to produce a resultant signal that appears to be random (or pseudo-random) in nature. Such a signal appears like noise to unauthorized receivers and discourages intelligible reception. The particular encryption algorithm used by the encryption device may be a proprietary algorithm, or may be based on a standard such as the Data Encryption Standard promulgated by the United States National Bureau of Standards.
To date, such secure communications have only been a feature available on conventional communications systems that make use of dedicated channels. This has occurred in part because the encrypted signal itself comprises a 12 thousand bit per second (KBS) data stream (D) that requires substantially all of the available spectrum of the assigned channel as depicted in FIG. 3. Such a signal presents compatibility problems when compared to the trunked channel maintenance protocol described above, and hence a combined secure and trunked system has not been forthcoming. Conventional communications systems alone have supported secure communications needs.
Conventional channel allocation systems, however, do not represent optimum usage of increasingly crowded communications spectrum. Trunked systems are well recognized to make more efficient usage of available channel allocations. At the same time, both government and industry continue to demand greater security in their communications services. Accordingly, there exists a strongly felt need for a combined secure and trunked communications system.